The histamine H4 receptor is a 390 amino-acid, seven-transmembrane G protein coupled receptor with approximately 40% homology to the histamine H3 receptor. In contrast to the H3 receptor, which is primarily located in the brain, the H4 receptor is expressed at greater levels in eosinophils and mast cells, among other inflammatory cells. H4 receptor ligands should thus be suitable for the treatment of various inflammatory disorders. Examples of diseases where treatment with H4 ligands is particularly appropriate are inflammatory bowel disease, Crohn's disease, colitis ulcerosa, dermatitis, psoriasis, conjunctivitis, rheumatoid arthritis, respiratory diseases such as adult respiratory distress syndrome, acute respiratory distress syndrome, bronchitis, chronic bronchitis, chronic obstructive pulmonary disease, cystic fibrosis, asthma, emphysema, rhinitis, chronic sinusitis, allergy, allergy-induced airway responses, allergic rhinitis, viral rhinitis, non-allergic rhinitis, perennial and seasonal rhinitis, nasal congestion and allergic congestion.
Recently some histamine H4 receptor ligands have been developed. An overview of the current advance in H4 ligand research and patenting is given in Expert Opin. Ther. Patents (2003) 13(6). Examples of Histamine H4 receptor ligands can be found in WO 02/072548, WO 04/022537 and in Terzioglu et al., J. Bioorg. Med. Chem. Left. 14 (2004), 5251-5256.
Although H4 ligands are known there is still a need to further provide new H4 ligands that are good drug candidates. In particular, preferred compounds should bind potently to the histamine H4 receptor whilst showing little affinity for other receptors. They should be well absorbed from the gastrointestinal tract, be metabolically stable and possess favourable pharmacokinetic properties. They should be non-toxic and demonstrate few side-effects.